Transformers used in electrical and electronic applications for "transforming" an input voltage to a higher or lower voltage (and often referred to as "Boost" and "Buck" converters respectively) are well known to persons skilled in the art. A problem with known transformers is to provide assemblies which operate with both continuous input and output currents. This is possible with a series of boost and buck converters, but a simple cascade of two such converters requires a substantial number of components and, additionally, requires complex circuitry to drive the devices. A known DC--DC converter which operate with continuous input and output currents is described in U.S. Pat. No. 4,184,197 (CUK et al) and is commonly referred to as a CUK converter. The CUK converter is a single ended converter which has a capacitor in series with the output to provide the continuous output current when the switching device is on. This capacitor is subjected to high ripple currents and is a critical and expensive component.